We propose to perform a prospective virologic study of HIV-1 infection among intravenous drug users (IVDU) representing largely minorities and women to define and quantify virus load in relation to immune status, and to identify prevalence and incidence of AZT resistance in an HIV-1 infected population. We will follow-up a seroprevalent cohort of 300 participants, divided equally by sex and matched for immune status over two years to describe the viral load by age, race, sex, drug use frequency and immunoactivation markers, such as B-2 microglobulin and/or neopterin. At baseline, a cross-sectional design will be used to compare levels of viral burden by measures of immune status (i.e., CD4 level). The cohort will be observed by two annual visits to describe trends in correlating between immune status, viral load and other variables. Similarly, the incidence of AZT resistance during the follow-up visits will be determined. The freeze- thaw effect on specimens used for viral quantitation will be assessed by comparing the data obtained from fresh samples vs samples frozen for one and two years. This evaluation is important for the use of frozen biologic repositories in non-concurrent prospective studies.